Magnesium-base alloy



nited States Patent MAGNESIUM-BASE ALLOY John C. McDonald, Midland, Mich., assignor to The Dow Chemical Company, Midland, Mich., a corporation of Delaware No Drawing. Application October 16, 1953, Serial No. 386,666

3 Claims. (Cl. 75-168) The invention relates to magnesium-base alloys. It more particularly concerns magnesium-base alloys containing thorium and having among other desirable properties a high creep strength.

Recent innovations particularly in aircraft in which stressed parts are subjected to elevated temperatures have made it desirable to provide metals of light weight which are resistant to creep. Although the magnesium-base alloys are among thte lightest of the structural metals, they have not met with wide acceptance in applications where stressed parts are exposed to elevated temperatures. According to the present invention, a magnesiumbase alloy composition is provided possessing the characteristic lightness of magnesium together with a high resistance to creep at elevated temperatures.

The invention is predicated upon the discovery that by alloying both manganese and thorium with magnesium in certain proportions, the alloy obtained possesses .both high creep strength and the lightness characteristic of magnesium. The invention then consists of the improved magnesium-base alloy herein fully described and particu* larly pointed out in the claims.

In accordance with the invention from 0.5 to 8 percent of thorium and from 0.2 to 2 percent of manganese by weight are alloyed with the magnesium. A preferred amount of thorium is between about 2 to 4 percent and that of manganese from 0.5 to 1.8 percent. The order in which the metals thorium and manganese are alloyed with the magnesium is immaterial. If desired, both metals may be alloyed at once with the magnesium. Alloying can be efiected by melting a suitable quantity of magnesium in a steel melting pot under the protection of a suitable saline flux, such as one free from magnesium chloride. An example of a suitatble saline flux is one composed of 57 parts of KCl, 28 parts of CaClz, 12.5 parts of BaClz, and 2.5 parts of CaFz. The molten magnesium may be held at about 700 C. while the alloying constituents are added.

In the case of the addition of manganese, this can be added as in the conventional manufacture of the binary magnesium-base magnesium-manganese alloy. The usual practice in this regard is to treat the molten magnesium melt with a reducible manganesecompound, such as manganese chloride. The molten magnesium reacts with the manganese chloride producing metallic manganese which alloys with the molten magnesium. If desired,

metalhc manganese may be alloyed directly with the magnesium by stirring the molten magnesium to which the metallic manganese is added in particulate form. Thorium may be added as metallic thorium, the melt being stirred until the thorium dissolves in it.

After alloying has been efiected, the melt is allowed to remain undisturbed for a time to permit separation of flux as well as unalloyed metal, if any, thereby obtaining a clean melt of the alloy. The settled alloy then may be decanted or otherwise transferred into a suitable mold, such as a sand or metal mold, to solidify.

The following tabulation of alloys made in accordance 2,774,664 Patented Dec. 18, 1956 with the foregoing procedure is illustrative of the invention.

A A A C A A A B B 1 A, cast test bars heat treated at 1,050 F. for 24 hours followed by heat treatment at 400 F. for 6 hours. B, extruded test bars heat treated at 400 F. for 16 hours. 0, rolled extruded stock heat treated at 300 F. for

=h t '300" F.

From Table 1, it is manifest that the ternary alloys of the present invention exemplified by alloys No. 5 to 9, inclusive, exhibit higher creep strength than the parent binary alloys of Mg-Th and Mg-Mn examples of which are alloys numbered 1 to 4, inclusive, of the same table.

The alloy may be rolled into sheet or plate or forged at temperatures between about 600 and 1000 F. A preferred rolling temperature is about 850 F. It is also preferable to roll extruded stock rather than cast metal stock. Extrusion of the cast alloy can be effected over about the same temperature range that is suitable for rolling, i. e. 600 to 1000 F. An example of an alloy within the scope of the invention having the nominal composition of 2.3 percent thorium, 1.3 percent manganese, the balance being magnesium, was cast into a billet and then extruded to form rolling stock. The ex truded stock so-obtained was rolled into sheet 0.045 inch thick having the following room temperature properties after annealing at the tabulated annealing temperatures:

Table 2 Tensile Yield Percent F. strength, strength, lonp. s. i. p. s. i. gation 1 Annealed for 1 hour at the indicated temperature.

The sharp decline in yield strength at about 700 F.

3. A magnesium-base alloy containing about'3 percent of thorium and about 1.2 percent of manganese, the balance being magnesium.

References Cited in the file of this patent UNITED STATES PATENTS 2,121,292 Haughton et al June 21, 1938 2,221,319 Altwicker Nov. 12, 1940 OTHER REFERENCES Journal of Metals, March 1952, pages 287294. Pub. by the A. I. M. E., New York. 

1. A MAGNESIUM-BASE ALLOY COMPRISING FROM 0.5 TO 8 PERCENT OF THORIUM, FROM 0.2 TO 2 PERCENT OF MANGANESE, THE BALANCE BEING MAGNESIUM. 